About lubricating oils

Automotive motor oil

Automotive motor oils are mineral oils (distillates of petroleum). These oils are specifically designed for the lubrication of internal combustion engines. Motor oils are rated (classified) primarily by three organizations: The Society of Automotive Engineers (SAE), American Petroleum Institute (API) and the International Lubricant Standardization and Approval Committee (ILSAC).

SAE grades motor oils by numerical values that refer to the viscosity range of a given oil grade. The viscosity of a liquid can be thought of as its "thickness" or a measure of its resistance to flow. This measure is often given in units of centistokes (cSt) and is more correctly called the Kinematic Viscosity. The numbers specified in an oil viscosity grade (ex: 10W30) represent the grade of the oil, not the weight. Although higher grades typically correlate to higher viscosities, a "10 grade" oil is not a "10 weight" oil.

Motor oils are either graded as single viscosity or multi viscosity oils. Most motor oils in use today are multi viscosity oils. For multi viscosity oils, the "W" means "winter". The number preceding the "W" is the SAE viscosity grade of the oil for cold temperatures. The number without the "W" refers to the grade of the oil at temperatures above 100 °C (212 °F). Viscosity index improvers are added to oils to make them multi-viscosity oils.

SAE motor oil grades:

SAE viscosity grade Minimum Kinematic viscosity (cSt)@100 °C Maximum Kinematic viscosity (cSt)@100 °C
5 3.8 Less than 4.1
10 4.1 Less than 5.6
20 5.6 Less than 9.3
30 9.3 Less than 12.5
40 12.5 Less than 16.3
50 16.3 Less than 21.9
60 21.9 Less than 26.1

API motor oil rating:

This rating is marked on a container of motor oil and designates the current service rating of the oil. The designation is shown on the API service symbol, which is a round symbol also showing the SAE oil grade.

As of this writing in 2011, the latest rating by the API was "SN". Ratings begin with "S" which denotes service for gasoline engines. The "N" is the level of service. Previous ratings are alphabetized (A,B,C, etc.) and are typically obsolete for the most current automobiles. However, a current service rated oil that exceeds a manufacturer's requirement for a particular service rating is a valid replacement. This is because the current service rating exceeds that of earlier ratings in performance.

API service ratings that designate CI or CJ are rated for diesel engines. However, some oils are rated for both diesel and gasoline engines and would therefore be marked as such.

ILSAC rating:

The ILSAC organization works with the API. ILSAC also has ratings for motor oil, but the ILSAC ratings include standards for fuel economy. The most current ILSAC rating is GF-5. The previous obsolete ratings are GF-4, GF-3, GF-2, and GF-1.

Automotive gear oil

A good note to begin with is that SAE viscosity grades for gear oil are not the same as motor oil. For example, a 75W90 gear oil is approximately equivalent in viscosity to 10W40 motor oil. Remember that the grade of an oil is not the weight of an oil, but rather a reference on a scale as to the viscosity range of an oil. As with motor oils, larger grade numbers do correspond to increased viscosity.

The primary difference between gear oils and motor oils is the use of additives to meet the specific requirements for a given application. Gear oils often have additives for extreme pressure applications. Additionally, gear boxes do not typically employ pumps as do engines , so the oil must be transported by the lubricated components themselves. There are exceptions, however. Some engines share oil with the transmission.

API transmission oil classifications:

Classification Use conditions Intended application
API GL-1 Light Bevel gears, worm gears, asynchronous manual transmissions
API GL-2 Moderate Worm gears
API GL-3 Moderate Bevel gears, truck transmissions without hypoid gears
API GL-4 Light to heavy Bevel gears, hypoid gears, truck transmissions, axles
API GL-5 Severe Hypoid gears, differentials
API GL-6 Obsolete N/A

Gear oil viscosity grades:

SAE viscosity grade Minimum Kinematic viscosity (cSt)@100 °C Maximum Kinematic viscosity (cSt)@100 °C
75 4.2 Less than 7
80 7 Less than 11
85 11 Less than 14
90 14 Less than 25
140 25 Less than 43
150 43

Synthetic oils

Synthetic oils are not artificial oils. Synthetic oils are derived from crude oil. The base oil is modified at the molecular level whereby the molecules are usually made larger for the purpose of achieving higher performance. According to Valvoline, synthetic oils are compatible with non synthetics and switching back and forth from a synthetic to a non-synthetic has no ill effects. Compatibility, however, should be constrained to the type of oil. For example, synthetic motor oil versus non-synthetic motor oil. The only disadvantage of synthetic oils is expense.

Automatic transmission fluids (ATF)

Like gear oils, automatic transmission fluids are designed to lubricate gears. Some vehicle models specify the use of ATF in manual transmissions and differentials. ATF, however, is specifically designed to meet the needs of hydraulic valves, clutches and other components of automatic transmissions. ATF is colored red or green so it can be easily distinguished from other types of oils.

Two stroke oil (also called two cycle oil)

There are many types of two cycle oil available. Two cycle oil may be mineral oil, vegetable oil, full synthetic or synthetic blend. Regardless of the type, two cycle oils are designed specifically to be burned with the gasoline (more correctly, fuel air mixture). Because of this, two cycle oil must produce little or no ash. The oil must adequately lubricate the combustion chamber and other related components (often, crankshaft bearings), while at the same time evacuate through the exhaust system cleanly. Conventional motor oils are not designed in this way because they are not intended to be mixed with the combustion fuel.

Suspension oil (hydraulic)

Suspension performance plays a vital role in power sports such as motocross and enduro racing. To succeed with proper performance, it makes sense that a suspension must be tuned properly. Many factors are involved that affect it, one of which is the oil being used. The demands on suspension oil differ from other lubricating applications. Suspension oil must lubricate internal components, but it also is used to control damping when forced through small orifices, valve shims and other small spaces. As this behavior takes place, heat is generated and there is also a risk of foaming, depending in part on the particular suspension design. The oil then must not only lubricate, it must remain stable so the suspension can remain consistent. This means the oil must carry heat for cooling, must resist foaming and must hold its designed viscosity for as long as possible. Viscosity is important not only for lubricating, but also for its affect on damping. Varying viscosity in a suspension is one alternative method for tuning damping.

Brake fluid (hydraulic)

Brake fluid, like suspension oil, is a type of hydraulic oil. Brake fluid undergoes extreme stress. Because of this, it must be designed to remain stable with no compressibility within a wide range of temperatures from extreme cold to extreme heat. Vaporization at high temperature must be prevented in a brake system, otherwise braking failure would result. It is of utmost importance that a vehicle manufacturers recommendation for brake fluid be strictly adhered to. In the United states, brake fluids are specified as DOT grades. DOT means Department Of Transportation. Each DOT rating specifies boiling points, viscosity limits and its primary constituent.

Conclusion

A lot more can be said about lubricating oils and there are in fact many more types of oil than are discussed here. The oils that are produced today have specific purposes that are intended to meet the requirements as specified by manufacturers for given applications. This article intends to give a brief overview of oils for vehicles in general, allowing readers to understand that it is important to choose the correct oil for the intended use. Where ever there is doubt, the most wise choice in oil selection is to follow a manufacturer's recommendation.

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